Die collet with cavity wall recess

ABSTRACT

The invention provides a method and apparatus for picking a separated semiconductor die 128 from a wafer and placing it on a die attach pad 144 for bonding thereto. In a preferred embodiment, the apparatus comprises a die collet having a body 124 with a proximal end 123 and a distal end 125; at least one pair of spaced-apart walls 136 extending distally from the distal end of the body and having opposing faces 181 defining an aperture 126, the faces of the walls being sloped such that a distal portion of the aperture is wider than the die and a proximal portion of the aperture is narrower than the die; a recess 152 on the faces of the walls extending substantially the length of the die, the recess having a distally-facing surface 154 for contacting at least a portion of the top side 138 of the die; and means for holding the die in the aperture, usually including a vacuum port 128. The method comprises providing a collet like the aforementioned; positioning the aperture 126 over a die 120; exerting a vacuum pressure through the vacuum port 128 to retain the die in the aperture such that a portion of the top side contacts the distally-facing surface 154 of the recess 152; positioning the die over the pad 144 such that the bottom side of the die is parallel to the pad; and discontinuing the vacuum pressure to release the die from the aperture.

BACKGROUND OF THE INVENTION

The present invention relates generally to bonding of semiconductor diesin integrated circuit (IC) packages, and more specifically to diecollets for picking separated semiconductor dies from a wafer andplacing the dies on a die attach pad, lead frame or seating plane towhich the die is bonded.

FIG. 1 illustrates a conventional technique for separating asemiconductor die from a silicon wafer. The wafer 10 is mounted onexpandable adhesive film 12 secured at its edges by clamps 14. Disposedbeneath adhesive film 12 is expansion frame 16 movable in the directionof arrow 18 such that expansion frame 16 pushes upward on film 12distending it vertically and stretching it horizontally, providing atense, flat surface on which wafer 10 resides.

Dies 20 are separated from wafer 10 by sawing cuts 22 at the edges ofeach die. The result is a plurality of edges 20, usually rectangular inshape, mounted on film 20 and separated by the saw kerf at cuts 22.

For placement of the dies on a die attach pad of an integrated circuit(IC) package, each die 20 is lifted from film 12 by a collet 24, whichis usually mounted at the end of a pick-and-place device, such as arobot arm or other mechanical actuator. Die collets for placement ofsemiconductor dies on die attach pads for bonding are known in the art,and are commercially available from, for example, Small Precision Toolsof Petaluma, Calif. Automated die attach systems employing die colletslike the aforementioned are also well known, being commercially sold by,for example, Advanced Mechanization Incorporated of Horsham, Penna.

As shown in FIG. 2, the collet 24 has an aperture 26 at its distal endand a vacuum hole 28 extending through the length of collet 24 throughwhich a vacuum pressure is exerted for lifting die 20. Aperture 26 hasfour sloped walls 30 which, at the distal end of aperture 26 are spacedapart a distance such that die 20 can fit within aperture 26.

As shown in FIGS. 3A and 3B, walls 30 of aperture 26 are sloped at anangle such that a gap 34 is disposed between the top of die 20 and endsurface 36 of aperture 26. Gap 34 is provided so as to avoid damagingthe top surface 38 of die 20. Thus, contact between collet 24 and die 20is limited to the corners 40 of die 20.

Referring again to FIG. 1, a die 20 is lifted by collet 24 from film 12by positioning aperture 26 over a die 20, exerting a vacuum pressure inthe direction of arrow 32 and simultaneously moving an ejector pin 42upwardly against the lower surface of film 12. Ejector pin 42 assistscollet 24 in removing die 20 from adhesive film 12. Collet 24 is thenmoved upwardly away from film 12, with die 20 preferably in a positionshown in FIG. 3A.

Collet 24 is then positioned over a die attach pad or lead frame wherethe die is to be bonded. As shown in FIG. 4, the die 20 is placed overpad 44 with a layer of bonding material 46, such as solder, disposedbetween die 20 and pad 44. Heat is then applied, usually through pad 44,so as to melt solder 46, bonding die 20 to pad 44.

Using this known technique of die placement, several significantproblems may arise. One such problem is shown in FIG. 5. As shown by thedotted line, die 20 can become tilted in aperture 26 of collet 24. Thiscan occur for various reasons, such as the upward force of ejector pin42 when the die is removed from film 12, the force of vacuum pressurethrough vacuum hole 28, the inertial force as the collet is moved inposition over a die attach pad, or the uneven surface of the solderlayer 46.

Whatever the reason, the result of the tilting of die 20 is shown inFIG. 6. Die 20 is bonded in a tilted position, with bonding material 46having a thickness greater on one side than on the other. The tilt ofdie 20 and the varied thickness of bonding material 46 may lead todevice failure during electrical test or during operation in the fieldbecause of the uneven thermal gradient across the die.

Another significant problem which arises in known processes is chippingor cracking of the corners of die 20. Such chipping or cracking can takethe form shown in FIG. 7, wherein corners 40 contacting walls 30 ofaperture 26 have been damaged either when the die 20 is lifted from film12, or when die 20 is bonded to pad 44. Further, chipping and crackingcan occur also at the lower corner 50 of die 20, in the illustration ofFIG. 6. This is thought to result from the concentration of stresses atcorner 50 when a tilted die in collet 24 is placed on pad 44.

A further problem results from the requirement that aperture 26 besealed at edges 40 of die 20 in order to provide a proper vacuum seal toretain die 20 in the aperture. Known collets suffer from poor sealing,and have a significant incidence of dropped dies.

Moreover, these problems with tilted and damaged die result in asignificant amount of machine down time in the manufacturing process.Currently, when a die is tilted in the aperture 26 of die collet 24, orcorners 40 of die 20 are chipped or cracked, either the die is placed onthe bonding pad and bonded in the tilted or cracked condition, or, ifthe machine operator notices the problem, the die attach machine must bestopped to adjust the position of the die so that it will be placedparallel to the die attach pad. A cracked or chipped die must bediscarded and replaced.

Accordingly, a method and apparatus for placing a die on a die attachpad or lead frame is desired which would (a) maintain the back surfaceof the die in a desired plane, (b) reduce or avoid die cracking and/orchipping on or around the corners and edges of the die, (c) achieve auniform thickness of bonding material between the die and the pad orlead frame, (d) reduce machine downtime attributed to correcting thetilting problems and recovering dropped dies, and (e) reduce the defectrate and test yield loss due to poor attachment of dies.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for placing asemiconductor die on an attachment pad, lead frame or seating plane ofan IC device. In one embodiment of the apparatus of the presentinvention, a collet is provided for placing a semiconductor die on anattachment pad, the die having a bottom side for attaching to the padand a top side opposite the bottom side, the collet comprising a bodyhaving a proximal end and a distal end; means disposed at the distal endof the body for maintaining the top side of the die in a position spacedapart from the distal end; means disposed at the distal end of the bodyfor maintaining the die in a position relative to the body in at leastone direction parallel to said pad; means disposed at the distal end ofthe body for maintaining the bottom side of the die in a plane parallelto said pad; and means for lifting the die.

In an exemplary embodiment, the means for maintaining the die in aposition relative to the body in at least one direction parallel to thepad comprises at least one pair of spaced apart walls extending from thedistal end of the body, the walls having opposing faces defining anaperture into which the top side of the die extends.

In a further embodiment, the means for maintaining the top side of thedie in a position spaced apart from the distal end of the body comprisesa slope on the faces of the walls such that a distal portion of theaperture is wider than the die and the proximal portion of the apertureis narrower than the die.

In still another embodiment, the means for maintaining the bottom sideof the die in a plane parallel to the pad comprises a recess disposed inthe faces of the walls and extending substantially the length of thedie, the recess having a distally-facing surface for contacting at leasta portion of the top side of the die.

In another aspect of the present invention, a method is provided forplacing a semiconductor device on an attachment surface for bondingthereto, the device having a bottom side for bonding to the attachmentsurface and a top side opposite the bottom side, the method comprisingthe steps of providing a collet having at least one pair of spaced apartwalls extending in a distal direction, the walls having opposing slopedfaces defining an aperture widest at its distal end, a recess disposedon said faces extending the length of the device and having adistally-facing surface, and a vacuum port disposed between the walls;positioning the aperture over a device; exerting a vacuum pressurethrough the vacuum port to retain the device in the aperture, a portionof the top side of the device contacting the distally-facing surface ofthe recess; positioning the device over the attachment surface such thatthe bottom side of the device is substantially parallel to the surface;and discontinuing the vacuum pressure to release the device from theaperture. The device may be a single semiconductor die, or a subassemblyhaving one or more dies and/or substrates mounted together.

The present invention provides a method and apparatus for placing asemiconductor device on an attachment surface which overcome several ofthe aforementioned problems of known die placement techniques. Inparticular, the apparatus and method of the present invention maintainthe device in a desired plane, eliminating the tilting problems of priordevices. In addition, the invention greatly reduces problems withcracking and/or chipping on or around the corners and edges of the dieassociated with known die placement methods. Further, an improved vacuumseal is obtained to reduce the incidence of dropped die. As a result,die can be bonded to an attachment surface with a desired planarorientation, a uniform thickness of bonding material and withoutcracking or chipping of the die. Machine downtime is significantlyreduced, and yield is substantially improved.

The invention and objects and features thereof will be more readilyapparent from the following detailed description and appended claimswhen taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a front cross-sectional view of a known die collet and wafermounting apparatus.

FIG. 2 is a perspective view of the die collet of FIG. 1.

FIGS. 3A and 3B are front cross-sectional and bottom elevational viewsof the die collet of FIG. 1.

FIG. 4 is a front cross-sectional view of a die bonded to a die attachpad.

FIG. 5 is a front cross-sectional view of the collet of FIG. 1.

FIG. 6 is a front cross-sectional view of a die bonded to a die attachpad.

FIG. 7 is a front cross-sectional view of a die.

FIGS. 8A and 8B are front cross-sectional and bottom elevational viewsof the die collet of the present invention.

FIGS. 9-14 front cross-sectional views of alternative embodiments of thedie collet of FIGS. 8A and 8B.

FIG. 15 is a front cross-sectional view of a die mounted to a die attachpad in accordance with the method of the present invention.

FIGS. 16A and 16B are front cross-sectional views of the die collet ofthe present invention lifting electronic sub-assemblies.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

Referring to FIGS. 8A and 8B, the die collet of the present inventionincludes a body 124 having a proximal end 123 that may have variousconfigurations depending upon the manner in which the collet isutilized. For example, proximal end 123 may be configured for couplingto a robotic arm or other actuator.

Distal end 125 of body 124 has four walls 130 extending distallytherefrom, walls 130 having opposing sloped faces 131 defining anaperture 126 therebetween. The faces 131 of walls 130 are spaced adistance from each other such that the top side 138 of die 120 fitswithin aperture 126 while bottom side 137 of die 120 is disposeddistally of the distal ends of walls 130. The slope angle θ of faces 131may be, for example, between 30° and 90°, but may be greater or lessdepending upon the application. A vacuum port 128 extends from proximalend 123 to distal end 125, the distal opening of vacuum port 128 lyingbetween walls 130.

A recess 152 is disposed on faces 131 of walls 130, the recess 152having a distally-facing surface 154 for contacting top surface 138 ofdie 120. Recess 152 including surface 154 is configured and positionedsuch that a gap 134 is maintained between top side 138 of die 120 andend surface 136 of aperture 126. Gap 134 ensures that top side 138 ofdie 120 is not damaged by contact with surface 136. Recess 152 extendsalong face 131 of each wall 130 a distance at least as long as thecorresponding dimension of die 120 such that all of corner edges 140 ofdie 120 lie within recess 152.

By contacting die 120 at a point interior to corners 140 on the top sideof the die, corner edges 140 are protected from chipping or cracking.Further, distally-facing surfaces 154 ensure that die 120 remains in adesired planar position, eliminating the tilting problems associatedwith known devices. In addition, the collet of the present inventionprovides an improved seal around the surface of die 120 when vacuumpressure is exerted through vacuum port 128, thereby reducing theincidence of dropped dies due to vacuum leakage from aperture 126.

In a preferred embodiment, the cross-section of recess 152, as shown inFIG. 8A, is generally round with an open portion through which corneredges 140 extend. In a second preferred embodiment, recess 152, asillustrated in FIG. 9, has an angular configuration, with surface 154being planar for contacting top side 138 of die 120, and a secondsurface 155 perpendicular to surface 154 and extending perpendiculartherefrom in a distal direction adjacent the edges of die 120. Thisembodiment provides a particularly reliable seal on top side 138 of die120 due to the larger area of contact between surface 154 and top side138. Alternatively, recess 152 may have one of the variousconfigurations illustrated in FIGS. 10-14.

It should be noted that while faces 131 of walls 130 are shown with aslope, the collet of the present invention may be provided with walls130 having faces 131 which are substantially straight and parallel withthe edges of die 120. The sloping faces 131 shown in the figures arepreferred, however, since such a sloping configuration allows highertolerance in the positioning accuracy required for positioning aperture126 over die 120.

In the method of the present invention, a collet as shown in FIG. 8A ispositioned over a die 120 such that the top side 138 extends intoaperture 126, with corner edges 140 disposed in recess 152. Proximal end123 of the collet may be coupled to a mechanical actuator or roboticarm. Vacuum pressure is exerted through vacuum port 128, drawing die 120toward body 124 of the collet with top side 138 of die 120 contactingsurface 154 of recess 152. Collet 124 is then positioned over a dieattach pad or lead frame 144, depending upon the application. Asillustrated in FIG. 15, a layer of bonding material, usually solder, isdisposed on the top surface of the die attach pad, the die 120 beingpositioned on top of and contacting the bonding material. Die 120 ismaintained in a horizontal and vertical position by walls 130 and thevacuum pressure through port 128, and is further maintained in a planeparallel to the die attach pad 144 by distally-facing surfaces 154 inrecess 152.

In a preferred embodiment, bonding material 146 is solder, which may besolder ribbon or wire of a tin-lead mixture, which is solid at roomtemperature. Solder material 146 is heated, usually by heating dieattach pad 144 using a heater block positioned thereunder, convertingsolder 146 to a molten state. Collet body 124 is then subject tomovement in a plane parallel to the top surface 148 of bonding pad 144,a technique called "scrubbing," believed to improve the intermetallicdiffusion between the die and the die attach pad. Usually, the scrubbingmotion is circular, and does not extend beyond the edges of die attachpad 44. Alternatively, the scrubbing motion can be a linear back-and-forth motion in one or more directions over the die attach pad.During the scrubbing motion, recesses 152 ensure that die 120 ismaintained in a fixed position relative to body 124 of the collet andremains parallel to bonding surface 48 of die attach pad 44.

After scrubbing, vacuum pressure through vacuum hole 128 is discontinuedand collet body 124 retracted away from die 120. Heating of solder 146is discontinued so that solder 146 returns to a solid state, therebybonding die 120 to die attach pad 144. The bonded die is now ready forsubsequent process steps.

As shown in FIG. 15, using the apparatus and method of the presentinvention, the bonded die is substantially parallel to die attach pad144, and thickness of bonding material 146 is uniform throughout thebonding area. Further, die 120 is left substantially intact, withoutcracks or chips at its corner or edge surfaces.

The invention is further suited to lifting and placing prefabricatedelectronic sub-assemblies on a surface for bonding thereto. Two examplesare shown in FIGS. 16A-16B. In FIG. 16A, a substrate 160 is mounted on afirst die 120, and a second die 162 is mounted on the substrate. In thisembodiment, recesses 152 are positioned such that gap 134 is largeenough to accommodate substrate 160 and second die 162 between first die120 and end surface 136 of the collet. In the example of FIG. 16B, oneor more dies 164 are mounted on the surface of a substrate 166, which islifted by the collet. Corners 168 of substrate 166 extend into recesses152 of the collet, thereby maintaining the horizontal and verticalalignment of the substrate. In this embodiment, the substrate is placedon another surface, such as a die attach pad or second substrate, towhich it is bonded. Various other types of sub-assemblies may also beplaced for bonding using the collet of the present invention.

While the invention has been described with reference to a specificembodiment, the description is illustrative of the invention and is notto be construed as limiting the invention. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A collet for placing a prefabricated electronicsub-assembly on an attachment surface, said subassembly comprising asubstrate having a bottom side for attaching to said attachment surfaceand a top side opposite said bottom side, and at least one die mountedon said top side of said substrate, said collet comprising:a body havinga proximal end and a distal end; a first surface recessed in said distalend, said first surface having a configuration to receive said topsurface of said substrate, whereby said bottom side of said substrate ismaintained in a plane parallel to said attachment surface; a secondsurface recessed in said distal end spaced from and parallel with saidfirst surface to accommodate said die mounted on said substrate; atleast one pair of spaced-apart walls extending from said distal end ofsaid body, said walls having opposing faces defining an aperture intowhich said top surface of said substrate extends, whereby saidsub-assembly is maintained in a position relative to said body in atleast one direction parallel to said attachment surface; and meansdisposed at said distal end of said body for holding said subassembly.2. The collet as in claim 1, wherein said first recessed surface extendssubstantially the length of said substrate.
 3. The collet of claim 1wherein said means for holding comprises a vacuum port disposed in saiddistal end of said body between said walls for exerting a vacuumpressure in said aperture.
 4. The collet of claim 1 further comprising asecond pair of spaced apart walls perpendicular to said first pair, saidtwo pairs of walls defining a four-sided aperture.
 5. The collet ofclaim 1, wherein a portion of said first surface is receded, wherebycontact between said first surface and said substrate at edgessurrounding said substrate is minimized.
 6. The collet of claim 5,wherein said recessed portion of said first surface is at leastpartially round in cross-section.
 7. The collet of claim 1, wherein saidfirst pair of walls is perpendicular to said first recessed surface,said walls extending distally from said first surface.
 8. A collet forplacing a prefabricated electronic sub-assembly on an attachmentsurface, said sub-assembly comprising a bottom die having a bottom sidefor attaching to said attachment surface and a top side opposite saidbottom side, a substrate having a bottom side and a top side, saidbottom side of said substrate attached to said top side of said bottomdie, and a top die mounted on said top surface of said substrate, thecollet comprising:a body having a proximal end and a distal end; a firstsurface recessed in said distal end, said first surface having aconfiguration to receive said top surface of said bottom die, wherebysaid bottom side of said bottom die is maintained in a plane parallel tosaid attachment surface; a second surface recessed in said distal endspaced from and parallel with said first surface to accommodate saidsubstrate and said top die; at least one pair of spaced-apart wallsextending from said distal end of said body, said walls having opposingfaces defining an aperture into which said top surface of said bottomdie extends, whereby said sub-assembly is maintained in a positionrelative to said body in at least one direction parallel to saidattachment surface; and means disposed at said distal end of said bodyfor holding said sub-assembly.
 9. The collet as in claim 8, wherein saidfirst recessed surface extends substantially the length of saidsubstrate.
 10. The collet of claim 8 wherein said means for holdingcomprises a vacuum port disposed in said distal end of said body betweensaid walls for exerting a vacuum pressure in said aperture.
 11. Thecollet of claim 8 further comprising a second pair of spaced apart wallsperpendicular to said first pair, said two pairs of walls defining afour-sided aperture.
 12. The collet of claim 8, wherein a portion ofsaid first surface is receded, whereby contact between said firstsurface and said bottom die at edges surrounding said bottom die isminimized.
 13. The collet of claim 12, wherein said recessed portion ofsaid first surface is at least partially round in cross-section.
 14. Thecollet of claim 8, wherein said first pair of walls are perpendicular tosaid first recessed surface, said walls extending distally from saidfirst surface.